Prenatal development

Prenatal development (from Latin natalis, meaning 'relating to birth') includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth. In human pregnancy, prenatal development is also called antenatal development. The development of the human embryo follows fertilization, and continues as fetal development. By the end of the tenth week of gestational age the embryo has acquired its basic form and is referred to as a fetus. The next period is that of fetal development where many organs become fully developed. This fetal period is described both topically (by organ) and chronologically (by time) with major occurrences being listed by gestational age. The very early stages of embryonic development are the same in all mammals. Later stages of development across all taxa of animals and the length of gestation vary. In the human: Different terms are used to describe prenatal development, meaning development before birth. A term with the same meaning is the 'antepartum' (from Latin ante 'before' and parere 'to give birth') Sometimes 'antepartum' is however used to denote the period between the 24th/26th week of gestational age until birth, for example in antepartum hemorrhage. The perinatal period (from Greek peri, 'about, around' and Latin nasci 'to be born') is 'around the time of birth'. In developed countries and at facilities where expert neonatal care is available, it is considered from 22 completed weeks (usually about 154 days) of gestation (the time when birth weight is normally 500 g) to 7 completed days after birth. In many of the developing countries the starting point of this period is considered 28 completed weeks of gestation (or weight more than 1000 g). Fertilization marks the first germinal stage of embryonic development. When semen is released into the vagina, the spermatozoa travel through the cervix and body of the uterus and into the fallopian tubes where fertilization usually takes place. Many sperm cells are released with the possibility of just one managing to adhere to and enter the thick protective layer surrounding the egg cell (ovum). The first sperm cell to successfully penetrate the egg cell donates its genetic material (DNA) to combine with the DNA of the egg cell resulting in a new organism called the zygote. The term 'conception' refers variably to either fertilization or to formation of the conceptus after its implantation in the uterus, and this terminology is controversial. The zygote will develop into a male if the egg is fertilized by a sperm that carries a Y chromosome, or a female if the sperm carries an X chromosome. The Y chromosome contains a gene, SRY, which will switch on androgen production at a later stage leading to the development of a male body type. In contrast, the mitochondrial DNA of the zygote comes entirely from the egg cell.